Could this have damaged my CPU?

I have an athlon II X4 640 and when I was running prime95 the temperature hit 59 degrees celcius and was at that temp for like 15 minutes or so. I reseated the cooler and improved airflow and it's maxing out at like 48 degrees. Could this brief period of such a high temperature have damaged it? I'm really not well-informed in what a safe CPU temp is.

No, you haven't even hurted it one bit, when a CPU reaches its temperature limits, it usually shuts the computer down to allow it to cool, or throttles down in terms of clock speeds to run cooler, CPU's have plenty of safety features so its unlikely you have damaged it_

Perhaps if you were running an insane amount of voltage, you might kill it, but usually the CPU will overheat quickly and turn the PC off or itself_

First of all, AMD CPU's have a limit of 62 celsius. They don't have as high a thermal limit as Intel processors. They also run cooler. Don't worry about the chip, those temps are perfectly safe and even if they weren't, 15 minutes is nowhere near long enough to degrade it.